The present invention relates, in general, to bonding of silicon wafers to another semiconducting or semi-insulating substrate and, more particularly, to an automated method for bonding a plurality of wafers at the same time.
Silicon wafer bonding, which is bonding a silicon wafer to another silicon wafer, is a useful replacement for conventional epitaxial processing. Unprocessed or partially processed silicon top wafers can be bonded to an insulating bottom wafer or substrate to form a silicon on insulator (SOI) structure. Some recent applications take advantage of the flexibility of wafer bonding to created devices in which both the top and bottom wafers are processed to provide devices with complex buried layer structures.
Bonding two wafers to each other is usually accomplished by bonding one wafer pair at a time using an apparatus similar to that shown in U.S. Pat. No. 4,883,215 issued to Goesele et al on Nov. 28, 1989. Bonding one wafer at a time, however, is a slow and expensive process. Moreover, prior single wafer bonders require manual handling of wafers resulting in dirty wafers and unpredictable yields. Further, previous manual wafer bonding method have relatively low productivity, resulting in increased costs for the useful and valuable bonded wafer substrates.
Another problem with previous wafer bonding methods is that the wafers were joined prior to permanent bonding in a horizontal orientation. The horizontal orientation causes dust and contamination to land on the wafers which is trapped during bonding. Trapped dust and contamination causes voids, which are also called bubbles, in the bonded wafer.
What is needed is an automated method for joining wafers that provides high productivity, low cost, and reproducible void-free bonded wafers. Moreover, a method of wafer bonding is needed that can be integrated with existing semiconductor wafer handling equipment to allow cassette-to-cassette transfer. Also, a method for joining wafers is needed that reduces particulate contamination problems associated with previous manual wafer bonding methods.